Elastic wave filter, and duplexer and electronic device using same

ABSTRACT

An elastic wave filter has first, second, and third IDT electrodes whose wiring electrodes are connected to unbalanced signal terminal; fourth IDT electrode disposed between the first and second IDT electrodes; fifth IDT electrode disposed between the second and third IDT electrodes; and first and second balanced signal terminals connected to the wiring electrodes of fourth and fifth IDT electrodes, respectively. The ground electrodes of the first through fifth IDT electrodes are connected to the ground. Signals in opposite phase and in phase with a signal input to unbalanced signal terminal are output from first and second balanced signal terminals, respectively. The wiring electrodes of second and third IDT electrodes are adjacent to the ground electrode of fifth IDT electrode. The wiring electrode of one of first and second IDT electrodes is adjacent to the wiring electrode of fourth IDT electrode. The ground electrode of the other is adjacent to the ground electrode of fourth IDT electrode. This configuration can suppress spuriousness in the bandpass.

This application is a U.S. national stage application of PCTInternational Application No. PCT/JP2009/00972, filed Mar. 4, 2009.

TECHNICAL FIELD

The present invention relates to an elastic wave filter for use in aportable phone, for example, and to a duplexer and an electronic deviceusing the elastic wave filter.

BACKGROUND ART

As shown in the configuration diagram of FIG. 14, a conventional elasticwave filter of this type has unbalanced signal terminal 16; and first,second, and third interdigital transducer (IDT) electrodes 17A, 17B, and17C whose wiring electrodes are electrically connected to unbalancedsignal terminal 16. The conventional elastic wave filter also has fourthIDT electrode 17D disposed between first and second IDT electrodes 17Aand 17B, and fifth IDT electrode 17E disposed between second and thirdIDT electrodes 17B and 17C. The conventional elastic wave filter alsohas first balanced signal terminal 18A electrically connected to thewiring electrode of fourth IDT electrode 17D, and second balanced signalterminal 18B electrically connected to the wiring electrode of fifth IDTelectrode 17E. In the conventional elastic wave filter, the groundelectrodes of first, second, third, fourth, and fifth IDT electrodes17A, 17B, 17C, 17D, and 17E are electrically connected to the ground. Inthe conventional elastic wave filter, a signal in opposite phase withthe signal input from unbalanced signal terminal 16 is output from firstbalanced signal terminal 18A, and a signal in phase with the signalinput from unbalanced signal terminal 16 is output from second balancedsignal terminal 18B. In the conventional elastic wave filter, the groundelectrodes of second and third IDT electrodes 17B and 17C are adjacentto the wiring electrode of fifth IDT electrode 17E, and the groundelectrodes of first and second IDT electrodes 17A and 17B are adjacentto the ground electrode of fourth IDT electrode 17D (see PatentLiterature 1, for example).

However, in such a conventional elastic wave filter, spuriousness occursin the bandpass.

That is, in the above conventional configuration, the ground electrodesof first and second IDT electrodes 17A and 17B are adjacent to theground electrode of fourth IDT electrode 17D, and thus spuriousness “S”occurs in the bandpass, as shown in the bandpass characteristics of FIG.15.

Further, an elastic wave filter where the bandpass characteristics areimproved by parallel-connecting the elastic wave filters of FIG. 14 isknown. As shown in FIG. 16, this conventional parallel-connected elasticwave filter has first longitudinally coupled resonator elastic wavefilter 5 and second longitudinally coupled resonator elastic wave filter9 formed on piezoelectric substrate 1. The first longitudinally coupledresonator elastic wave filter has first unbalanced signal terminal 2,and first balanced signal terminal 3 and second balanced signal terminal4 electrically connected to first unbalanced signal terminal 2. Thesecond longitudinally coupled resonator elastic wave filter has secondunbalanced signal terminal 6, and third balanced signal terminal 7 andfourth balanced signal terminal 8 electrically connected to secondunbalanced signal terminal 6.

First unbalanced signal terminal 2 and second unbalanced signal terminal6 are electrically connected to each other. First balanced signalterminal 3 and second balanced signal terminal 4 are electricallyconnected to first input/output terminal 10. Third balanced signalterminal 7 and fourth balanced signal terminal 8 are electricallyconnected to second input/output terminal 11. The input/output signalfrom first input/output terminal 10 is 180° out of phase with theinput/output signal from second input/output signal 11 (see PatentLiterature 2, for example).

However, in such a conventional parallel-connected elastic wave filter,the insertion loss degradation is large in the high-frequency part ofthe band.

FIG. 17 is a chart showing bandpass characteristics of a conventionalparallel-connected elastic wave filter. That is, in the conventionalparallel-connected elastic wave filter, as shown in FIG. 17,spuriousness “S” occurs in the high-frequency part of desired bandpassPB0. As a result, the insertion loss degradation is increased.

Patent Literature 1

-   Japanese Patent Unexamined Publication No. 2001-313540

Patent Literature 2

-   Japanese Patent Unexamined Publication No. 2002-314371

SUMMARY OF THE INVENTION

The present invention is directed to provide an elastic wave filtercapable of suppressing the occurrence of spuriousness and reducing theinsertion loss degradation, and a duplexer and an electronic deviceusing the elastic wave filter.

The present invention has the following elements:

-   -   an unbalanced signal terminal;    -   first, second, and third (interdigital transducer) IDT        electrodes whose wiring electrodes are electrically connected to        the unbalanced signal terminal;    -   a fourth IDT electrode disposed between the first and second IDT        electrodes;    -   a fifth IDT electrode disposed between the second and third IDT        electrodes;    -   a first balanced signal terminal electrically connected to the        wiring electrode of the fourth IDT electrode; and    -   a second balanced signal terminal electrically connected to the        wiring electrode of the fifth IDT electrode.        The respective ground electrodes of the first, second, third,        fourth, and fifth IDT electrodes are electrically connected to        the ground. A signal in opposite phase with a signal input from        the unbalanced signal terminal is output from the first balanced        signal terminal. A signal in phase with the signal input from        the unbalanced signal terminal is output from the second        balanced signal terminal. The respective wiring electrodes of        the second and third IDT electrodes are adjacent to the ground        electrode of the fifth IDT electrode. The wiring electrode of        one of the first and second IDT electrodes is adjacent to the        wiring electrode of the fourth IDT electrode. The ground        electrode of the other of the first and second IDT electrodes is        adjacent to the ground electrode of the fourth IDT electrode.

This configuration can suppress spuriousness in the bandpass.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic top view of an elastic wave filter in accordancewith a first exemplary embodiment of the present invention.

FIG. 2 is a schematic top view of an elastic wave filter in accordancewith another example of the first exemplary embodiment.

FIG. 3 is a chart showing bandpass characteristics of the elastic wavefilter in accordance with the first exemplary embodiment.

FIG. 4 is a chart showing bandpass characteristics of the elastic wavefilter of FIG. 2.

FIG. 5 is a schematic top view of a parallel-connected elastic wavefilter in accordance with a second exemplary embodiment of the presentinvention.

FIG. 6 is a chart showing bandpass characteristics of theparallel-connected elastic wave filter in accordance with the secondexemplary embodiment.

FIG. 7 is a schematic top view of a parallel-connected elastic wavefilter in accordance with a third exemplary embodiment of the presentinvention.

FIG. 8 is a top view showing a specific example of theparallel-connected elastic wave filter in accordance with the thirdexemplary embodiment.

FIG. 9 is a perspective view of an essential part of theparallel-connected elastic wave filter of FIG. 8.

FIG. 10 is a schematic top view of a parallel-connected elastic wavefilter in accordance with a fourth exemplary embodiment of the presentinvention.

FIG. 11 is an electric circuit diagram of a duplexer in accordance witha fifth exemplary embodiment of the present invention.

FIG. 12 is a chart showing bandpass characteristics in accordance withthe fifth exemplary embodiment.

FIG. 13 is a configuration diagram of an electronic device in accordancewith a sixth exemplary embodiment of the present invention.

FIG. 14 is a configuration diagram of a conventional elastic wavefilter.

FIG. 15 is a chart showing bandpass characteristics of the conventionalelastic wave filter.

FIG. 16 is a configuration diagram of a conventional parallel-connectedelastic wave filter.

FIG. 17 is a chart showing bandpass characteristics of the conventionalparallel-connected elastic wave filter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, the present invention is described in accordance withexemplary embodiments with reference to the accompanying drawings. Thepresent invention is not limited by the following exemplary embodiments.

First Exemplary Embodiment

FIG. 1 is a schematic top view of an elastic wave filter in accordancewith the first exemplary embodiment of the present invention. Withreference to FIG. 1, the elastic wave filter of this exemplaryembodiment has unbalanced signal terminal 12; and first, second, andthird interdigital transducer (IDT) electrodes 13A, 13B, and 13C whosewiring electrodes are electrically connected to unbalanced signalterminal 12. The elastic wave filter of this embodiment also has fourthIDT electrode 13D disposed between first and second IDT electrodes 13Aand 13B. The elastic wave filter of this embodiment also has fifth IDTelectrode 13E disposed between second and third IDT electrodes 13B and13C. The elastic wave filter of this embodiment also has first balancedsignal terminal 14A electrically connected to the wiring electrode offourth IDT electrode 13D, and second balanced signal terminal 14Belectrically connected to the wiring electrode of fifth IDT electrode13E. In the elastic wave filter of this embodiment, the groundelectrodes of first, second, third, fourth, and fifth IDT electrodes13A, 13B, 13C, 13D, and 13E are electrically connected to the ground. Inthe elastic wave filter of this embodiment, a signal in opposite phasewith a signal input from unbalanced signal terminal 12 is output fromfirst balanced signal terminal 14A. In the elastic wave filter of thisembodiment, a signal in phase with the signal input from unbalancedsignal terminal 12 is output from second balanced signal terminal 14B.In the elastic wave filter of this embodiment, the wiring electrodes ofsecond and third IDT electrodes 13B and 13C are adjacent to the groundelectrode of fifth IDT electrode 13E. In the elastic wave filter of thisembodiment, the wiring electrode of one of first and second IDTelectrodes 13A and 13B is adjacent to the wiring electrode of fourth IDTelectrode 13D. In the elastic wave filter of this embodiment, the groundelectrode of the other of first and second IDT electrodes 13A and 13B isadjacent to the ground electrode of fourth IDT electrode 13D.

Specifically, each of first, second, third, and fifth IDT electrodes13A, 13B, 13C, and 13E has five electrode fingers. Fourth IDT electrode13D has six electrode fingers. The electrode fingers of each of first,second, third, fourth, and fifth IDT electrodes 13A, 13B, 13C, 13D, and13E are formed of a wiring electrode and a ground electrode. In firstIDT electrode 13A, the ground electrode is disposed on the outermostside such that this ground electrode is adjacent to the ground electrodein fourth IDT electrode 13D. The number of electrode fingers of fourthIDT electrode 13D is an even number. Thus, the electrode finger on theopposite side of the ground electrode adjacent to first IDT electrode13A is a wiring electrode. This wiring electrode of fourth IDT electrode13D is electrically connected to first balanced signal terminal 14A, andis adjacent to the wiring electrode of second IDT electrode 13B.

In second IDT electrode 13B, the number of electrode fingers is an oddnumber. Thus, the electrode finger on the opposite side of the wiringelectrode adjacent to fourth IDT electrode 13D is a wiring electrode.This wiring electrode is adjacent to the ground electrode of fifth IDTelectrode 13E. The wiring electrode of fifth IDT electrode 13E iselectrically connected to second balanced signal terminal 14B.

In fifth IDT electrode 13E, the number of electrode fingers is an oddnumber. Thus, the electrode finger on the opposite side of the groundelectrode adjacent to second IDT electrode 13B is also a groundelectrode. This ground electrode is adjacent to the wiring electrode ofthird IDT electrode 13C.

Reflector 15A is disposed on the opposite side of fourth IDT electrode13D with respect to first IDT electrode 13A. Reflector 15B is disposedon the opposite side of fifth IDT electrode 13E with respect to thirdIDT electrode 13C.

Such a configuration can suppress the occurrence of spuriousness in thebandpass.

FIG. 3 is a chart showing bandpass characteristics of the elastic wavefilter in accordance with this exemplary embodiment. In FIG. 3,characteristics 301 shown by the solid lines are those of this exemplaryembodiment, and characteristics 302 shown by the dotted lines are those(the same as shown in FIG. 15) of a conventional elastic wave filter. Asshown in the “A” part of FIG. 3, for characteristics 301 of the elasticwave filter of this exemplary embodiment, spuriousness “S” appearing inconventional characteristics 302 has disappeared in the bandpasscharacteristics.

Further, as shown in the “B” part of FIG. 3, the attenuationcharacteristics in characteristics 301 (of this exemplary embodiment)can be improved, in comparison with those in (conventional)characteristics 302, by using at least one of the following threeconfigurations.

The first configuration is related to a pitch spacing between theelectrode fingers in first, second, third, fourth, and fifth IDTelectrodes 13A, 13B, 13C, 13D, and 13E. This is a configuration where,with respect to center line A-AA in second IDT electrode 13B, pitchspacing P1 on one side and pitch spacing P2 on the other side areasymmetric.

The second configuration is related to a pitch gradation of theelectrode fingers in first, second, third, fourth, and fifth IDTelectrodes 13A, 13B, 13C, 13D, and 13E. This is a configuration where,with respect to center line A-AA in second IDT electrode 13B, the pitchgradation of pitch spacings P1 on one side and the pitch gradation ofpitch spacings P2 on the other side are asymmetric. In thisconfiguration, for example, pitch spacings P1 in the pitch gradation onone side are gradually reduced by spacing α as the distance from centerline A-AA is increased, and pitch spacings P2 in the pitch gradation onthe other side are gradually reduced by spacing β (≠α) as the distancefrom center line A-AA is increased.

The third configuration is related to spacing between first, second,third, fourth, and fifth IDT electrodes 13A, 13B, 13C, 13D, and 13E.This is a configuration where at least one of spacing L14 between firstIDT electrode 13A and fourth IDT electrode 13D and spacing L42 betweenfourth IDT electrode 13D and second IDT electrode 13B is different fromat least one of spacing L25 between second IDT electrode 13B and fifthIDT electrode 13E and spacing L53 between fifth IDT electrode 13E andthird IDT electrode 13C.

In this exemplary embodiment, second IDT electrode 13B has an odd numberof electrode fingers. Thus, the ground electrode of first IDT electrode13A is adjacent to the ground electrode of fourth IDT electrode 13D, andthe wiring electrode of fourth IDT electrode 13D is adjacent to thewiring electrode of second IDT electrode 13B. However, as shown in FIG.2, second IDT electrode 13B may have an even number of electrodefingers. In this case, the wiring electrode of first IDT electrode 13Ais adjacent to the wiring electrode of fourth IDT electrode 13D, and theground electrode of fourth IDT electrode 13D is adjacent to the groundelectrode of second IDT electrode 13B.

However, the configuration of FIG. 1 is preferable in terms ofimprovement in attenuation characteristics. FIG. 4 is a chart showingbandpass characteristics of the elastic wave filter of FIG. 2. In FIG.4, characteristics 402 of the elastic wave filter configured as shown inFIG. 2 is superimposed on characteristics 401 of the elastic wave filterconfigured as shown in FIG. 1. As obvious from FIG. 4, attenuationcharacteristics in characteristics 401 are improved in comparison withthose in characteristics 402. Therefore, a more preferable configurationwhere attenuation characteristics are further improved is that of FIG.1, where second IDT electrode 13B has an odd number of electrodefingers, the ground electrode of first IDT electrode 13A is adjacent tothe ground electrode of fourth IDT electrode 13D, and the wiringelectrode of fourth IDT electrode 13D is adjacent to the wiringelectrode of second IDT electrode 13B.

With a configuration where the arrangement of the electrode fingers ofthe elastic wave filter of FIG. 1 is vertically inverted, the sameadvantages can be obtained. In this case, the connection relationsbetween unbalanced signal terminal 12, first balanced signal terminal14A, second balanced signal terminal 14B, and respective electrodefingers are the same as those shown in FIG. 1. With this configuration,the ground electrodes of second and third IDT electrodes 13B and 13C areadjacent to the wiring electrode of fifth IDT electrode 13E, the wiringelectrode of one of first and second IDT electrodes 13A and 13B isadjacent to the wiring electrode of fourth IDT electrode 13D, and theground electrode of the other of first and second IDT electrodes 13A and13B is adjacent to the ground electrode of fourth IDT electrode 13D.

Second Exemplary Embodiment

In the elastic wave filter of the second exemplary embodiment of thepresent invention, the bandpass characteristics can be further improvedby parallel-connecting the elastic wave filters described in the firstexemplary embodiment.

A parallel-connected elastic wave filter of this exemplary embodiment isdescribed with reference to the accompanying drawings. As shown in FIG.5, the parallel-connected elastic wave filter of this exemplaryembodiment has piezoelectric substrate 21, and first longitudinallycoupled resonator elastic wave filter 25 and second longitudinallycoupled resonator elastic wave filter 29 formed on piezoelectricsubstrate 21. The first longitudinally coupled resonator elastic wavefilter has first unbalanced signal terminal 22, first balanced signalterminal 23, and second balanced signal terminal 24. Each of firstunbalanced signal terminal 22, first balanced signal terminal 23, andsecond balanced signal terminal 24 is electrically connected to firstlongitudinally coupled resonator elastic wave filter 25. The secondlongitudinally coupled resonator elastic wave filter has secondunbalanced signal terminal 26, third balanced signal terminal 27, andfourth balanced signal terminal 28. Each of second unbalanced signalterminal 26, third balanced signal terminal 27, and fourth balancedsignal terminal 28 is electrically connected to second longitudinallycoupled resonator elastic wave filter 29.

First unbalanced signal terminal 22 and second unbalanced signalterminal 26 are electrically connected to each other. First balancedsignal terminal 23 and third balanced signal terminal 27 areelectrically connected to first input/output terminal 30. Secondbalanced signal terminal 24 and fourth balanced signal terminal 28 areelectrically connected to second input/output terminal 31.

The input/output signal from first input/output terminal 30 is 180° outof phase, i.e. in opposite phase, with the input/output signal fromsecond input/output terminal 31. Actually, in consideration of designaccuracy and variations, the phase difference between the input/outputsignal from first input/output terminal 30 and the input/output signalfrom second input/output terminal 31 is in the range of 180°±10°.

First longitudinally coupled resonator elastic wave filter 25 also hasfirst, second, third, fourth, and fifth IDT electrodes 32, 33, 34, 35,and 36. The first longitudinally coupled resonator elastic wave filterhas grating reflectors 42 and 43 at both ends of the arrangement offirst, second, third, fourth, and fifth IDT electrodes 32, 33, 34, 35,and 36 in the propagation direction of elastic waves. Similarly, secondlongitudinally coupled resonator elastic wave filter 29 has sixth,seventh, eighth, ninth and tenth IDT electrodes 37, 38, 39, 40, and 41.The second longitudinally coupled resonator elastic wave filter hasgrating reflectors 44 and 45 at both ends of the arrangement of sixth,seventh, eighth, ninth and tenth IDT electrodes 37, 38, 39, 40, and 41in the propagation direction of elastic waves.

One end of each of first, third, and fifth IDT electrodes 32, 34, and 36is electrically connected to first unbalanced signal terminal 22, andthe other end thereof is connected to the ground. Similarly, one end ofeach of sixth, eighth, and tenth IDT electrodes 37, 39, and 41 iselectrically connected to second unbalanced signal terminal 26, and theother end thereof is connected to the ground.

That is, as shown in FIG. 5, in each of first, third, and fifth IDTelectrodes 32, 34, and 36, one plurality of commonly connected electrodefingers faces the other plurality of commonly connected electrodefingers such that the former electrode fingers are disposed alternatelywith the latter electrode fingers. The one plurality of commonlyconnected electrode fingers in each of first and fifth IDT electrodes 32and 36 and the one plurality of commonly connected electrode fingers inthird IDT electrode 34 are electrically connected to first unbalancedsignal terminal 22. The other plurality of commonly connected electrodefingers in each of first and fifth IDT electrodes 32 and 36 and theother plurality of commonly connected electrode fingers in third IDTelectrode 34 are connected to the ground. As shown in FIG. 5, each ofsixth, eighth, and tenth IDT electrodes 37, 39, and 41 has one pluralityof commonly connected electrode fingers and the other plurality ofcommonly connected electrode fingers. The one plurality of commonlyconnected electrode fingers in each of sixth and tenth IDT electrodes 37and 41, and the one plurality of commonly connected electrode fingers ineighth IDT electrode 39 are electrically connected to second unbalancedsignal terminal 26. The other plurality of commonly connected electrodefingers in each of sixth and tenth IDT electrodes 37 and 41, and theother plurality of commonly connected electrode fingers in eighth IDTelectrode 39 are connected to the ground.

One end of second IDT electrode 33 is connected to the ground, and theother end thereof is electrically connected to first balanced signalterminal 23. One end of seventh IDT electrode 38 is connected to theground, and the other end thereof is electrically connected to thirdbalanced signal terminal 27. As described above, first balanced signalterminal 23 and third balanced signal terminal 27 are electricallyconnected to first input/output terminal 30.

On the other hand, one end of fourth IDT electrode 35 is connected tothe ground, and the other end thereof is electrically connected tosecond balanced signal terminal 24. One end of ninth IDT electrode 40 isconnected to the ground, and the other end thereof is electricallyconnected to fourth balanced signal terminal 28. As described above,second balanced signal terminal 24 and fourth balanced signal terminal28 are electrically connected to second input/output terminal 31.

That is, as shown in FIG. 5, in each of second, fourth, seventh, andninth IDT electrodes 33, 35, 38, and 40, one plurality of commonlyconnected electrode fingers faces the other plurality of commonlyconnected electrode fingers such that the former electrode fingers aredisposed alternately with the latter electrode fingers. At one end ofeach of second and seventh IDT electrodes 33 and 38, the one pluralityof commonly connected electrode fingers is connected to the ground. Insecond IDT electrode 33, the other plurality of commonly connectedelectrode fingers is electrically connected to first balanced signalterminal 23. In seventh IDT electrode 38, the other plurality ofcommonly connected electrode fingers is electrically connected to thirdbalanced signal terminal 27. In each of fourth and ninth IDT electrodes35 and 40, the one plurality of commonly connected electrode fingers isconnected to the ground. In fourth IDT electrode 35, the other pluralityof commonly connected electrode fingers is electrically connected tosecond balanced signal terminal 24. In ninth IDT electrode 40, the otherplurality of commonly connected electrode fingers is electricallyconnected to fourth balanced signal terminal 28.

The input/output signal from first balanced signal terminal 23 is inopposite phase with the input/output signal from second balanced signalterminal 24. Further, the input/output signal from third balanced signalterminal 27 is in opposite phase with the input/output signal fromfourth balanced signal terminal 28.

The input/output signal from first balanced signal terminal 23 is inphase with the input/output signal from third balanced signal terminal27. Further, the input/output signal from second balanced signalterminal 24 is in phase with the input/output signal from fourthbalanced signal terminal 28.

With this configuration, the input/output signal from first input/outputterminal 30 is in opposite phase with the input/output signal fromsecond input/output terminal 31.

In this exemplary embodiment, this configuration can suppressspuriousness in the high-frequency part of the desired band, and thusreduce the insertion loss degradation.

FIG. 6 is a chart showing bandpass characteristics of theparallel-connected elastic wave filter in accordance with this exemplaryembodiment. The horizontal axis shows a frequency and the vertical axisshows bandpass characteristics of the filter at the frequency. FIG. 6shows characteristics 201 of this exemplary embodiment andcharacteristics 202 of a conventional example. As shown in FIG. 6, whendesired bandpass PB is the frequency range of 2.11 GHz to 2.17 GHz,spuriousness “S” in the high-frequency part thereof is suppressed andthus the insertion loss degradation is reduced in characteristics 201(shown by the thick line) of this exemplary embodiment. Specifically,while the insertion loss at a frequency of 2.17 GHz in thehigh-frequency part is 1.8 dB in characteristics 202 (shown by the thinline) of the conventional example, that in characteristics 201 of thisexemplary embodiment is improved to 1.4 dB.

As piezoelectric substrate 21 shown in FIG. 5, a piezoelectric substratehaving piezoelectric properties can be used. Examples of thepiezoelectric substrate include a Y-cut X-propagation LiNbO₃ substrate,and a LiNbO₃ substrate, LiYaO₃ substrate, and crystal substrate havingother cut angles.

Usable materials of the electrodes formed on piezoelectric substrate 21are metals, such as aluminum, or alloys thereof.

Each of first and second longitudinally coupled resonator elastic wavefilter 25 and 29 of this exemplary embodiment has five IDT electrodesarranged in the propagation direction of elastic waves. However, thenumber of IDT electrodes is not limited to this value as long as oneunbalanced signal can be converted into two balanced signals.

The electrodes in first and second longitudinally coupled resonatorelastic wave filters 25 and 29 of this exemplary embodiment are designedto have an identical configuration. However, the configuration of theelectrodes does not need to be identical as long as each longitudinallycoupled resonator elastic wave filter can convert one unbalanced signalinto two balanced signals.

It is preferable that the polarity of the electrode fingers in IDTelectrodes 32 and 36 at the outermost ends of IDT electrodes 32 through36 in first longitudinally coupled resonator elastic wave filter 25 isequal to the polarity of the electrode fingers in IDT electrodes 37 and41 at the outermost ends of IDT electrodes 37 through 41 in secondlongitudinally coupled resonator elastic wave filter 29. Such polaritycan be obtained by making the combination of the comb-shaped electrodesof respective IDT electrodes 32 through 36 in first longitudinallycoupled resonator elastic wave filter 25 identical with the combinationof the comb-shaped electrodes of respective IDT electrodes 37 through 41in second longitudinally coupled resonator elastic wave filter 29.

That is, the combination of the comb-shaped electrodes is made identicalin the pairs of IDT electrodes 32 and 37, IDT electrodes 33 and 38, IDTelectrodes 34 and 39, IDT electrodes 35 and 40, and IDT electrodes 36and 41. This configuration can make the input/output signal from firstbalanced signal terminal 23 in phase with the input/output signal fromthird balanced signal terminal 27, and the input/output signal fromsecond balanced signal terminal 24 in phase with the input/output signalfrom fourth balanced signal terminal 28. Thus, the parallel-connectedelastic wave filter of this exemplary embodiment can be implementedeasily.

Overlapping width L1 in first longitudinally coupled resonator elasticwave filter 25 is set larger than overlapping width L2 in secondlongitudinally coupled resonator elastic wave filter 29. Thisconfiguration can effectively distribute the frequencies at whichtransverse-mode spuriousness occurs.

Third Exemplary Embodiment

FIG. 7 is a schematic top view showing a parallel-connected elastic wavefilter in accordance with the third exemplary embodiment of the presentinvention. In the parallel-connected elastic wave filter of thisexemplary embodiment, as shown in FIG. 7, capacitance component 51 isinterposed between first input/output terminal 30 and secondinput/output terminal 31 of the parallel-connected elastic wave filterof the second exemplary embodiment.

That is, first balanced signal terminal 23 and third balanced signalterminal 27 are connected to first junction point 46. Second balancedsignal terminal 24 and fourth balanced signal terminal 28 are connectedto second junction point 47. First junction point 46 and third balancedsignal terminal 27 are connected by first wiring 48. Second junctionpoint 47 and second balanced signal terminal 24 are connected by secondwiring 49.

The configuration of FIG. 7 can be obtained as specifically shown inFIG. 8 and FIG. 9, for example. FIG. 8 is a top view of a specificconfiguration of FIG. 7, and FIG. 9 is a perspective view of anessential part thereof. That is, first wiring 48 and second wiring 49are crossed each other on piezoelectric substrate 21. Between firstwiring 48 and second wiring 49, dielectric film 50 is formed. With thisconfiguration, capacitance component 51 can be interposed between firstinput/output terminal 30 and second input/output terminal 31, as shownin FIG. 7.

With this configuration, the impedance with respect to firstinput/output terminal 30 can be matched to the impedance with respect tosecond input/output terminal 31.

Preferably, silicon oxide is used as dielectric film 50. Since a siliconoxide film can be manufactured at a low temperature, damage to theelements can be avoided. Further, the silicon oxide film works asdielectric film 50 having high precision and quality whose thickness iseasily controllable.

Dielectric film 50 covers the top surface of at least one of firstlongitudinally coupled resonator elastic wave filter 25 and secondlongitudinally coupled resonator elastic wave filter 29. Further,preferably, the dielectric film serves as a functional film of at leastone of first longitudinally coupled resonator elastic wave filter 25 andsecond longitudinally coupled resonator elastic wave filter 29.

For example, specifically, dielectric film 50 interposed between firstwiring 48 and second wiring 49 covers first, second, third, fourth, andfifth IDT electrodes 32, 33, 34, 35, and 36 and/or sixth, seventh,eighth, ninth, and tenth IDT electrodes 37, 38, 39, 40, and 41, andserves as a functional film thereof.

That is, a functional film of silicon oxide, for example, is formed onfirst, second, third, fourth, fifth, sixth, seventh, eighth, ninth, andtenth IDT electrodes 32, 33, 34, 35, 36, 37, 38, 39, 40, and 41, forexample. Thus, the functional film can serve as a protective film of theIDT electrodes. Further, the film can suppress unnecessary spuriousnessoccurring in the vicinity of the resonance frequency, and improvefrequency-temperature characteristics. This functional film is extendedto the portion between first wiring 48 and second wiring 49 in aposition different from that of IDT electrodes, so that the functionalfilm can also be used as dielectric film 50. Thus, the productivity canbe enhanced.

When LiTaO3 is used for the piezoelectric substrate, unnecessaryspuriousness occurring in the vicinity of the resonance frequency issmall. Thus, the functional film and dielectric film 50 may be formed incommon, using a resin-based material having high processability, such aspolyimide.

Fourth Exemplary Embodiment

FIG. 10 is a schematic top view showing a parallel-connected elasticwave filter in accordance with the fourth exemplary embodiment of thepresent invention. In the parallel-connected elastic wave filter of thisexemplary embodiment, as shown in FIG. 10, inductance component 52 isinterposed between first input/output terminal 30 and secondinput/output terminal 31 of the parallel-connected elastic wave filterof the second exemplary embodiment.

For example, a laminated inductor or a thin-film inductor is disposedbetween first input/output terminal 30 and second input/output terminal31 so as to be electrically connected thereto at both ends. Thereby,inductance component 52 is interposed between the input/outputterminals.

Inductance component 52 interposed between first input/output terminal30 and second input/output terminal 31 enables impedance matching. Thus,the capacitance value necessary for the impedance matching can bereduced. As a result, the crossing area between first wiring 48 andsecond wiring 49 as shown in FIG. 8 and FIG. 9 can be reduced, and thefilter can be downsized.

Fifth Exemplary Embodiment

Hereinafter, a description is provided for a duplexer in accordance withthe fifth exemplary embodiment of the present invention, with referenceto the accompanying drawings. FIG. 11 is an electric circuit diagram ofthe duplexer in accordance with this exemplary embodiment.

As shown in FIG. 11, diplexer 56 of this exemplary embodiment hastransmission elastic wave filter 53, reception elastic wave filter 54,and phase-shift circuit 55 electrically connected to transmissionelastic wave filter 53 and reception elastic wave filter 54. Receptionelastic wave filter 54 of this exemplary embodiment is formed of aparallel-connected elastic wave filter described in the second exemplaryembodiment.

FIG. 12 shows bandpass characteristics of an antenna duplexer forWideband Code Division Multiple Access (WCDMA) that includes duplexer56. FIG. 12 shows transmission bandpass characteristics 801 fortransmission bandpass PB1 and reception bandpass characteristics 802 forreception bandpass PB2. FIG. 12 shows that an excellent insertion lossof approximately −2.1 dB is achieved in reception bandpass PB2 (2.11 GHzto 2.17 GHz).

In this manner, a parallel-connected elastic wave filter described inthe second exemplary embodiment is used as at least one of receptionelastic wave filter 54 and transmission elastic wave filter 53. Thereby,duplexer 56 having a low insertion loss can be obtained.

Sixth Exemplary Embodiment

FIG. 13 is a configuration diagram of an electronic device in accordancewith the sixth exemplary embodiment of the present invention. In thisexemplary embodiment, as the electronic device, a configuration ofportable phone 90 is shown. With reference to FIG. 13, a receptionsignal at a frequency of 2.0 GHz received by antenna 91 is assigned tothe reception side by duplexer 92, and sent to elastic wave filter 94 ina first stage via low-noise amplifier 93. The signal whose noise isremoved by elastic wave filter 94 is sent to mixer 95, where thefrequency of the signal is reduced to 130 MHz. Then, the signal is sentto elastic wave filer 96 in a second stage. In elastic wave filter 96, asignal is taken out with a low insertion loss. Thus, a voice signal canbe heard via demodulator circuit 97 from speaker 98.

On the other hand, a voice input to microphone 99 is converted into adigital signal by A/D converter 100, phase-modulated by modulator 101,and input to mixer 102. The signal whose frequency is increased by mixer102 is input to elastic wave filter 103. The signal whose noise isremoved by elastic wave filter 103 is transmitted, as a transmissionsignal at a frequency of 2.0 GHz, via power amplifier 104 and duplexer92, from antenna 91.

In this manner, a parallel-connected elastic wave filter described inthe second exemplary embodiment is used as elastic wave filters 92, 94,96, and 103 in portable phone 90, for example. Thereby, portable phone90 having a low insertion loss can be obtained. That is, the listeningquality of portable phone 90 can be improved.

INDUSTRIAL APPLICABILITY

The present invention has advantages of suppressing spuriousness in ahigh-frequency part of the band and reducing the insertion lossdegradation, and thus is useful in various electronic devices, such as aportable phone.

The invention claimed is:
 1. An elastic wave filter comprising: anunbalanced signal terminal; a first interdigital transducer (IDT)electrode, a second IDT electrode, and a third IDT electrode whosewiring electrodes are electrically connected to the unbalanced signalterminal; a fourth IDT electrode disposed between the first IDTelectrode and the second IDT electrode; a fifth IDT electrode disposedbetween the second IDT electrode and the third IDT electrode; a firstbalanced signal terminal electrically connected to a wiring electrode ofthe fourth IDT electrode; and a second balanced signal terminalelectrically connected to a wiring electrode of the fifth IDT electrode,wherein respective ground electrodes of the first IDT electrode, thesecond IDT electrode, the third IDT electrode, the fourth IDT electrode,and the fifth IDT electrode are electrically connected to a ground,wherein a signal in opposite phase with a signal input from theunbalanced signal terminal is output from the first balanced signalterminal, wherein a signal in phase with the signal input from theunbalanced signal terminal is output from the second balanced signalterminal, wherein the respective wiring electrodes of the second IDTelectrode and the third IDT electrode are adjacent to the groundelectrode of the fifth IDT electrode, wherein the wiring electrode ofone of the first IDT electrode and the second IDT electrode is adjacentto the wiring electrode of the fourth IDT electrode, wherein the groundelectrode of the other of the first IDT electrode and the second IDTelectrode is adjacent to the ground electrode of the fourth IDTelectrode, wherein concerning a pitch spacing between electrode fingersin each of the first IDT electrode, the second IDT electrode, the thirdIDT electrode, the fourth IDT electrode, and the fifth IDT electrode,with respect to a center line in the second IDT electrode, the pitchspacings in the pitch gradation on one side are gradually reduced byspacing a as the distance from the center line increases and the pitchspacings in the pitch gradation on the other side are gradually reducedby spacing β as the distance from the center line increases, where α andβ are different.
 2. The elastic wave filter of claim 1, wherein thesecond IDT electrode has an odd number of electrode fingers, wherein theground electrode of the first IDT electrode is adjacent to the groundelectrode of the fourth IDT electrode, and wherein the wiring electrodeof the fourth IDT electrode is adjacent to the wiring electrode of thesecond IDT electrode.
 3. The elastic wave filter of claim 1, whereinconcerning a pitch spacing between electrode fingers in each of thefirst IDT electrode, the second IDT electrode, the third IDT electrode,the fourth IDT electrode, and the fifth IDT electrode, with respect to acenter line in the second IDT electrode, the pitch spacing on one sideand the pitch spacing on the other side are asymmetric.
 4. The elasticwave filter of claim 1, wherein at least one of a spacing between thefirst IDT electrode and the fourth IDT electrode and a spacing betweenthe fourth IDT electrode and the second IDT electrode is different fromat least one of a spacing between the second IDT electrode and the fifthIDT electrode and a spacing between the fifth IDT electrode and thethird IDT electrode.
 5. An elastic wave filter comprising: an unbalancedsignal terminal; a first IDT electrode, a second IDT electrode, and athird IDT electrode whose wiring electrodes are electrically connectedto the unbalanced signal terminal; a fourth IDT electrode disposedbetween the first IDT electrode and the second IDT electrode; a fifthIDT electrode disposed between the second IDT electrode and the thirdIDT electrode; a first balanced signal terminal electrically connectedto a wiring electrode of the fourth IDT electrode; and a second balancedsignal terminal electrically connected to a wiring electrode of thefifth IDT electrode, wherein respective ground electrodes of the firstIDT electrode, the second IDT electrode, the third IDT electrode, thefourth IDT electrode, and the fifth IDT electrode are electricallyconnected to a ground, wherein a signal in opposite phase with a signalinput from the unbalanced signal terminal is output from the firstbalanced signal terminal, wherein a signal in phase with the signalinput from the unbalanced signal terminal is output from the secondbalanced signal terminal, wherein the respective ground electrodes ofthe second IDT electrode and the third IDT electrode are adjacent to thewiring electrode of the fifth IDT electrode, wherein the wiringelectrode of one of the first IDT electrode and the second IDT electrodeis adjacent to the wiring electrode of the fourth IDT electrode, whereinthe ground electrode of the other of the first IDT electrode and thesecond IDT electrode is adjacent to the ground electrode of the fourthIDT electrode, wherein concerning a pitch gradation of pitch spacingsbetween electrode fingers in each of the first IDT electrode, the secondIDT electrode, the third IDT electrode, the fourth IDT electrode, andthe fifth IDT electrode, with respect to a center line in the second IDTelectrode, the pitch spacings in the pitch gradation on one side aregradually reduced by spacing α as the distance from the center lineincreases and the pitch spacings in the pitch gradation on the otherside are gradually reduced by spacing β as the distance from the centerline increases, where α and β are different.
 6. The elastic wave filterof claim 5, wherein the second IDT electrode has an odd number ofelectrode fingers, wherein the ground electrode of the first IDTelectrode is adjacent to the ground electrode of the fourth IDTelectrode, and wherein the wiring electrode of the fourth IDT electrodeis adjacent to the wiring electrode of the second IDT electrode.
 7. Theelastic wave filter of claim 5, wherein concerning a pitch spacingbetween electrode fingers in each of the first IDT electrode, the secondIDT electrode, the third IDT electrode, the fourth IDT electrode, andthe fifth IDT electrode, with respect to a center line in the second IDTelectrode, the pitch spacing on one side and the pitch spacing on theother side are asymmetric.
 8. The elastic wave filter of claim 5,wherein at least one of a spacing between the first IDT electrode andthe fourth IDT electrode and a spacing between the fourth IDT electrodeand the second IDT electrode is different from at least one of a spacingbetween the second IDT electrode and the fifth IDT electrode and aspacing between the fifth IDT electrode and the third IDT electrode.